Behavioral Patterns «Prev

Command Pattern in Java

Illustrate how the Command Pattern decouples request senders (invokers) from request receivers.

The Command Pattern encapsulates a request as an object, allowing you to parameterize clients with different requests, queue or log them, and support undo operations. In this example, we build a Switch that can control multiple devices (a Light and a Fan) without depending on their concrete interfaces.

Problem

Devices such as lights and fans have different interfaces (turnOn() vs startRotate()). A single switch should be able to control either device, but it cannot depend directly on their implementations.

Solution

The Command Pattern introduces a Command interface with an execute() method. Concrete command classes wrap the receiver objects and translate generic Switch requests into device-specific actions.

Java Implementation

package com.java.behavioral.command;

public interface Command {
    void execute();
}

class Light {
    public void turnOn() { System.out.println("Light is on"); }
    public void turnOff() { System.out.println("Light is off"); }
}

class Fan {
    public void startRotate() { System.out.println("Fan is rotating"); }
    public void stopRotate() { System.out.println("Fan is not rotating"); }
}
class LightOnCommand implements Command {
    private Light light;
    public LightOnCommand(Light l) { light = l; }
    public void execute() { light.turnOn(); }
}

class LightOffCommand implements Command {
    private Light light;
    public LightOffCommand(Light l) { light = l; }
    public void execute() { light.turnOff(); }
}

class FanOnCommand implements Command {
    private Fan fan;
    public FanOnCommand(Fan f) { fan = f; }
    public void execute() { fan.startRotate(); }
}

class FanOffCommand implements Command {
    private Fan fan;
    public FanOffCommand(Fan f) { fan = f; }
    public void execute() { fan.stopRotate(); }
}
class Switch {
    private Command upCommand, downCommand;
    public Switch(Command up, Command down) {
        this.upCommand = up;
        this.downCommand = down;
    }
    public void flipUp() { upCommand.execute(); }
    public void flipDown() { downCommand.execute(); }
}

Test Program


  public class TestCommand {
    public static void main(String[] args) {
        Light light = new Light();
        Switch lightSwitch = new Switch(
            new LightOnCommand(light),
            new LightOffCommand(light)
        );
        lightSwitch.flipUp();
        lightSwitch.flipDown();

        Fan fan = new Fan();
        Switch fanSwitch = new Switch(
            new FanOnCommand(fan),
            new FanOffCommand(fan)
        );
        fanSwitch.flipUp();
        fanSwitch.flipDown();
    }
}

Program Output

Light is on
Light is off
Fan is rotating
Fan is not rotating

Key Takeaways